IN A PLOT which sets up a contest between the wills and skills of the president of the United States and Libyan strongman Muammar al-Qaddafi, the leading character turns out to be a detective first grade in the New York Police Department, Angelo Rocchia.

The Fifth Horseman is a novel about thermonuclear bombs in the hands of terrorists. Whence this title? Those who are up on chapter six of The Revelation of Saint John may recall that white, red, black and pale horses had riders who might be called Pestilence, War, Famine and Death. These were known as "The Four Horsemen of the Apocalypse," enshrined many years ago in the title of a book by Blasco Ibanez. "Now . . . a Fifth Horseman has emerged from the entrails of hell to scourge humanity with terror, with arms so terrible even John's halucinating imagination could not have conceived them," to quote from this book.

In the genre of The Crash of '79 and The Day of the Jackal, this suspense novel is readable and in places exciting. It mixes fact and fiction -- names, places, events and details -- so relentlessly that a word of caution is in order. The reader had best regard the entire book as fictional and not attempt to judge which details are fact and which are fiction. If he does attempt this, he is bound to form an exaggerated opinion of the technological prowess of our country and the extent to which a deity looks after our affairs.

But for some, especially Washington readers, it will be intriguing to note which officials in this book are given their real names and which have new identities. The secretary of state, Andrew Peabody, is fictional. Warren Christopher, his deputy, gets his real name. Harold Brown is secreatry of defense and William Webster is director of the FBI. The head of the CIA, however, is Gardiner "Tap" Bennington, said to be a devotee of Allen Dulles. (Since when did that distinguished director of Central Intelligence spell his name "Allan"?) One is bound to wonder whether the authors felt that certain of these individuals had more assured tenure than others, and if so, what crystal ball they used. Providentially, the president is not given a name.

It would spoil the story to say more about the plot than that the terrorists have smuggled a hydrogen bomb into Manhattan to blackmail the president. But it is fair to note that inhabitants of greater New York will find themselves thoughtfully figuring how they would evacuate that great city on short notice. It is the old shouting-"Fire"-in-a-crowded-theater syndrome, only on a far more massive and agonizing scale. In Washington a little-known organization called the Federal Emergency Management Agency is responsible for such matters. The reader will quickly come to the prayerful hope that his agency is on top of its job and among other things is checking its shelters dutifully. How reassuring is it to know that as of 1980 FEMA's "new civil defense policy" has among its programs one which includes "planning for population relocation during times of international crisis as well as be adaptable to help deal with natural disasters and other peacetime emergencies"?

In a setting of power, advanced technology and international negotiation, the one figure in the book who has true-to-life identity and emotions is our New York cop, Angelo Rocchia. His street smarts, his experience, his personal woes make him stand out among the plastic figures who dot the landscape. Perhaps he belongs to the wrong generation, but the treatment he receives from his paramour, Grace Knowland, a reporter for The New York Times, makes one wonder about certain modern attitudes. Professionally, however, he is what every citizen wants in a detective, and he may be forgiven his resentment of the FBI since most big-city cops share it.

The authors, Larry Collins and Dominique Lapierre, former journalists for Newsweek and Paris match respectively, have collaborated on five books of which this is the first novel. Their initial effort, Is Paris Burning? was an exciting recreation of life in the French capital during World War II as seen through the eyes of intensely human participants. It is perhaps for this reason that the French characters and settings in The Fifth Horseman have a particularly solid resonance.

The publishers of this book could hardly have counted on the recent publicity given to Billy Carter's arrangements with the Libyan government. From the point of view of sales it is almost too good to be true. But don't let this distract us from the fact that Libya in the words of President Carter, "has repeatedly provided support for acts of international terrorism." In an unclassified study a few years ago the Central Intelligence Agency made the judgment that "Colonel al-Qaddafi has been one of the world's least inhibited practitioners of international terrorism."

George F. Will, the columnist, wrote a year ago, after an international conference on terrorism in Jerusalem: "When a government, such as that of Libya, is involved in terrorism from Ulster to Isreal, then only prudential considerations on the part of the nations attacked can weigh against actions to change that government. This subject comes . . . under the heading of thinking the unthinkable. But the beginning of wisdom in dealing with terrorism is to face this fact: no act is unthinkable when so many terrible acts are successful."

It may perhaps give a shudder to the readers of The Fifth Horseman to hear another judgment written in the same CIA study noted above: "The prospect of nuclear-armed terrorists can, in fact, no longer be dismissed. But because of the major problems that would be involved in the acquisition, storage, transport, and employment of a nuclear device, a more likely scenario -- at least in the short term -- would be a terrorist seizure of a nuclear weapons storage facility or a nuclear power plant to exploit the publicity and the bargaining power inherent in the attendant threat of radiological pollution."

The plot of this book is by no means as farfetched as it may at first appear. The combination of nuclear power and terrorism has indeed created a "fifth horseman" to manace man's survival. The implications are sobering in the extreme.